1. Field of the Invention
The present invention relates to a method in a data transmission system according to the preamble of the appended claim 1. Furthermore, the invention relates to a wireless data transmission system according to the preamble of the appended claim 11.
2. Brief Description of Related Developments
The considerable increase in the use of information services especially as a result of the increase in Internet services and so-called World Wide Web (WWW) services has generated a need to develop faster data transmission services to transmit information between the information service provider and the terminal using the information service. Furthermore, most information services contain so-called multimedia information, such as images, video image and sound. The transmission of such multimedia information requires a high data transmission rate to implement the highest possible real-time degree in the data transmission.
Data transmission systems intended for office use, i.e. so-called local area networks (LAN) are primarily implemented as landline systems. Thus, the communication between the terminals and the server is implemented either electrically via a cable or optically via an optical fibre. The advantage of such a fixed system is that it is possible to achieve relatively high data transmission rates. The disadvantage of such a fixed data transmission network is that it is difficult to make changes and the terminals have to be placed relatively close to the coupling points intented for them, wherein the mobility of the terminal is affected. The implementation of such a fixed local area network in an existing building is not always successful, or it is expensive to install the cables afterwards. On the other hand, especially in older buildings, the existing data transmission cables are not necessarily suitable for fast data transmission.
There are several wireless data transmission systems under development to implement local area networks. Numerous landline data transmission systems are based on the use of radio signals in the data transmission. One such data transmission system for a local area network that is being developed and is based on radio data transmission is a so-called HIPERLAN (HIgh PErformance Radio Local Area Network). The term broadband radio access network (BRAN) is also used for such a radio network.
In the version 2 of the HIPERLAN data transmission system that is under development, the aim is to reach a data transmission rate in the order of 25 Mbit/s when the maximum connection distance is some tens of metres. Such a system is suitable to be used within the same building for example as an internal local area network for a single office. There is also a so-called HIPERACCESS data transmission system under development, the aim of which is to reach the same data transmission rate as in said HIPERLAN/2 data transmission system, but a connection distance of a few hundreds of meters, wherein the HIPERACCESS system is suitable to be used as a regional local area network for example in schools and larger building complexes.
The appended FIG. 1b shows in a reduced manner the frame structure applied in the data link layer DLC of the HIPERLAN/2 system used as an example. The data frame FR is composed of control fields C, such as RACH (Random Access Channel), BCCH (Broadcast Control CHannel) and FCCH (Frequency Correction CHannel), and a data field D which comprises a fixed amount of time slots TS1, TS2, . . . , TSn, in which the actual useful information can be transmitted.
In the HIPERLAN/2 system, data transmission is based on time division multiple access TDMA, wherein there may be several simultaneous connections on the same channel, but each connection is allocated a time slot of its own in said frame, in which time slot information is transmitted. Because the data transmission quantity is not constant in every simultaneous connection, but varies temporally, a so-called adapting TDMA method is used in which the number of time slots allocated for each connection can vary from zero to maximum depending on the loading situation at the time and on the data transmission capacity allocated for the connection.
For the time division multiple access to work, the terminals coupled to the same node have to be synchronized with each other and with the transmission from the node. This can be achieved for example in such a way that the receiver of the wireless terminal receives signals on a channel. If a signal is not detected on the channel, the receiver changes over to receive on another channel, until all the channels have been examined or a channel is found on which a signal transmitted by an access point is detected. By receiving and demodulating this signal, it is possible to determine the moment of transmission of the control channel BCCH of the access point in question and to synchronize the terminal on the basis thereof. In some cases, the terminal can detect the signal of more than one access point, wherein the terminal advantageously selects the access point which has the strongest signal in the receiver, and performs the synchronization with this access point.
When the terminal is synchronized with the access point, the terminal can initiate a connection set-up to couple to this access point. This can be conducted advantageously in such a way that the terminal transmits on the RACH control channel a connection set-up request to the access point. In practice, this means that the terminal transmits in the time slot allocated to the RACH control channel and at the same time the access point listens to the communication on the channel i.e. receives signals on the channel frequency it is using. When the access point detects that a terminal is transmitting a connection set-up request message, it performs the procedures necessary for the connection set-up, such as resource allocation for the connection, if it is possible. In the resource allocation, the quality of service requested for the connection is taken into account, which affects e.g. the number of time slots to be allocated for the connection. The access point informs the terminal whether the connection set-up is possible or not. If the connection set-up is successful, the access point transmits in the BCCH control field e.g. data on the transmission time slots, reception time slots, connection identifier, etc. which are allocated for the connection. The number of transmission and reception time slots is not necessarily the same, because in several cases the quantity of information to be transmitted is not the same in both directions. For example when using an Internet browser, the quantity of information transmitted from the terminal is considerably smaller than the amount of information received in the terminal. Thus, with respect to the terminal, the required number of transmission time slots is smaller than that of reception time slots.
Furthermore, the number of time slots allocated for the connection can advantageously vary in different frames according to the need to transmit data. The access point controller is provided with a so-called scheduler, one function of which is the aforementioned allocation of time slots for different connections. The scheduler is implemented advantageously as an application program in the access point controller.
Since duplex data transmission is necessary in local area networks, duplex data transmission is also necessary on the radio channel. In a time division system this can be implemented either in such a way that some of the time slots of the frame are allocated for transmission from the wireless terminal to the access point (uplink) and some of them are allocated for the transmission from the access point to the wireless terminal (downlink), or in such a way that a separate frequency band is allocated for each data transmission direction. The HIPERLAN/2 system suggests the use of the former of the aforementioned methods, wherein the access point and the wireless terminals coupled to it do not transmit simultaneously.
In the HIPERLAN/2 systems, the access points can select the channel to be used in the connection irrespective of the other access points. Furthermore, the scheduler of the access point selects the moment of time to be used for the transmission irrespective of the other access points. In practice, this means that two or more access points can make an attempt to transmit simultaneously on the same channel, wherein the transmission is unsuccessful. In order to prevent this collision of transmissions, the transmitting access point or wireless terminal first listens to the communication of that channel on which the transmission is conducted. If no communication is detected on the channel within a given period of time, it is presumed that the channel is free and the transmission can be started. However, if communication is detected on the channel, the receiver is synchronized with this transmission. When the transmission is terminated, a possible new message is waited for and after that it is possible to retry to access the channel. However, several access points and/or wireless terminals may be waiting for their transmission turn, wherein situations may occur where several devices make an attempt of transmitting simultaneously. Furthermore, situations may arise in which all the devices on a particular channel are waiting for a transmission turn, i.e. the channel is underutilized. On the other hand, information is not transmitted in every time slot in every frame, wherein during such an empty time slot the channel is underutilized, because any other device waiting for transmission cannot transmit either during such a time slot in the present system.
As was presented earlier in this description, the radio local area network can comprise several access points, the scope of whose service area is influenced by the transmission capacity, ambient conditions, possible obstructions in the path of the signal, the directional pattern of the antenna, etc. In practice, the boundaries of the service area of the access points cannot be clearly defined, but the service areas of the access points located in the vicinity of each other overlap one another at least partly. Thus, in some cases, the wireless terminal can be located within the service area of two or more access points, but these access points do not necessarily detect the transmissions of each other and cannot synchronize with each other. The wireless terminal selects one of these access points to be used in the data transmission connection. On the other hand, the access point used in the connection at a given time can be changed when the wireless terminal is moved or when the quality of the connection varies, which is known as such. Because the access points can select the channel to be used in the connection irrespective of the other access points, and schedule the transmissions independently, it is possible that the properties of the data transmission between a wireless terminal and an access point used in the connection at a given time are influenced by one or more access points within whose service area the wireless terminal is located. Such interference can also occur in situations in which the frequencies to be used are not the same, but two access points transmit for example on adjacent channels.
The access points that interfere with the data transmission can also be access points of another radio network. This is possible especially in cases when in the same office building there are several radio network systems utilizing frequency ranges which overlap each other at least partly.
One criterion for setting up a connection between a wireless terminal and an access point is the quality of service (QoS) desired for the connection. In some connections, for example in data connections, the rate of the data transmission is not as significant a criterion as the reliability of the data transmission. Thus, the data transmission parameters are selected in such a way that as reliable a data transmission as possible is achieved. On the other hand, for example when transmitting an audio and video signal, it is the real-time quality of the data transmission that is important, not the accuracy. For such a connection requiring real-time data transmission, it is possible to allocate several time slots in a frame, wherein a higher data transmission rate can be achieved. If necessary, it is also possible to allocate several channels for one data transmission connection.
When the data transmission is being set up, the wireless terminal is listening in order to find out which access points have signals to be received. The wireless terminal advantageously measures the strength of the signals and selects the access point whose signal is the strongest at the moment. Thereafter the wireless terminal and the access point conduct connection set-up signalling for instance to transmit parameters such as the required data transmission rate, connection type, data transmission channel, time slots and connection identifier which are used in the connection.
Typically also during the connection, the wireless terminal measures the strength of the signal of the access point used in the connection as well as the strength of the signals of the other possible access points within the coverage area. If it is detected that the signal strength of another access point is sufficiently greater than the signal strength of the access point used at that particular moment, it is possible to conduct a handover to this access point, which is known as such.
The purpose of the present invention is to attain more effective utilization of the radio resources than in radio network systems of prior art. Another purpose of the invention is to produce a more disturbance-free data transmission system when compared with data transmission systems of prior art. The method according to the present invention is characterized in what will be presented in the characterizing part of the appended claim 1. The wireless data transmission system according to the present invention will be characterized in what will be presented in the characterizing part of the appended claim 11.
The invention is based on the idea that two allocation strategies are determined, one of which is selected at a given time. To select the strategy, it is examined whether there is other traffic on the channel. If other traffic is to be detected, it is examined which allocation strategy is selected, and an allocation strategy different from that of the connection being examined is selected for the connection to be set up. However, if traffic is not detected, it is possible to select the allocation strategy freely. In the first allocation strategy, the allocation of the time slots in the data field is initiated from the first starting point of the data field, and in the second allocation strategy the allocation of the time slots in the data field is initiated from the second starting point of the data field.
With the present invention, considerable advantages are achieved when compared with methods and wireless data transmission systems of prior art. By means of the method according to the invention, it is possible to increase the utilization ratio of each data transmission channel. Furthermore, in the data transmission system according to the invention, the effect of interference is reduced to a level which is lower than that in wireless data transmission systems of prior art. In the data transmission system according to the invention, the synchronization of the access points can be attained in a relatively simple manner without complex algorithms.
The implementation of practical applications is also facilitated when the two allocation strategies according to the invention are used in such a way that in the first allocation strategy the allocation of the time slots in the data field is initiated from the beginning of the data field, and in the second allocation strategy the allocation of the time slots is initiated from the end of the data field, and that the allocation strategy information is transmitted in a data frame.